There is a need in the explosive industry for small primers containing from 5-25 grams and even higher amounts of explosive material which is sensitive to a No. 6 cap as well as to detonating cord. Primers of this type must have sufficient physical strength, reliability and stability to be used as an extension of cap or detonating cord initiation system. Explosives, cast in a hot melt condition, and molds have been used and are being used today for this purpose. In some cases, the molds also serve as the package for the primer material.
A castable sensitive composition known as pentolite has long been used to prepare the larger sized primers. In some cases, pentolite has been used to prepare primers of very small design. The most commonly used pentolite is a blend of 50% TNT and 50% PETN. This is known as 50/50 pentolite. When heated to the melting point of the TNT (ca. 80.4.degree. C.), and above, the pentolite mixture is a fluid slurry of molten TNT with a large portion of insoluble coarse granular PETN in suspension. Rapid stirring is required to maintain the solid PETN in even suspension. Pentolite is usually cast at a temperature in the range of 90.degree.-100.degree. C. and when casting such pentolite the solid coarse PETN has a tendency to settle out, resulting in a higher concentration of PETN at the bottom of the mold. This condition leads to excessively high sensitivity in the area of high PETN concentration and low sensitivity in the area of low PETN concentration.
In addition, castings made of pentolite shrink excessively on cooling. Shrinkage usually results in a large conical surface cavity that must be filled in with a second topping-off with poured hot melt pentolite to fill the mold. With larger castings, several pourings may be necessary. Even with a casting weighing only 5 grams, topping-off is required.
Another difficulty in using pentolite is due to the fact that the melt solidifies or fudges quickly upon contact with colder surfaces. This property makes it difficult to fill molds completely as large void spaces are frequently entrapped in the casting, especially if the casting is small and of complicated design.
Pentolite is normally hand poured or valved from a hot melt tank. In each case, the melt must be agitated rather rapidly to keep the coarse granular PETN in suspension. Blockage by granular PETN can be a problem in feed systems. When blockage does occur in a valve or pump, the hazard of inadvertent dentonation is present due to the sensitivity of PETN to crushing impact and friction.
The above disadvantages apply to pentolite which is made by adding coarse granular PETN to molten TNT, adding coarse granular PETN to molten TNT suspended in water and cooled to produce granular pentolite, or coprecipitating PETN and TNT from an acetone solution poured into water.
In summary, the problems associated with the use of 50/50 pentolite are:
1. rapid solidification
2. settling of coarse granular PETN
3. hazards associated with high concentration of granular PETN in melting tanks, pumps, valves and pipes
4. shrinkage